Amplifying the Temperature Sensitivity of Zero-Field Splitting in Mn(II) Through Ligand Tuning

Controlling the temperature response of magnetic resonance properties is an essential step toward novel molecular thermome-ters or quantum sensing platforms. To date, however, demonstrations of using molecular design to control the temperature dependence of zero-field splitting (D), a critical property that governs the electron paramagnetic resonance (EPR) response of open-shell molecules, have been absent in the literature. Herein we demonstrate that ligand design can control the temperature dependence of D. To do so, we prepared and analyzed three different encapsulated Mn2+ complexes. High-field, high-frequency EPR spectroscopy reveal EPR spectra for all complexes that vary in width as a function of temperature, indicating a change in D value. At lower temperatures, these temperature sensitivities change substantially with ligand shell and are stark, ranging from 2.2 to 9.8 MHz/K. These results are the first demonstration of the ability to tune the variable-T nature of D via ligand selection in any complex, the first for the Mn2+ metal ion, and demonstrate higher sensitivities than the NV center of diamond (74 kHz/K).